Editing Placoderm (section)

{{Short description|Extinct paraphyletic class of fishes}}
{{Automatic taxobox
| name = Placoderms
| fossil_range = {{geological range|439|358.9|latest=recent}}Late [[Llandovery epoch|Llandovery]] – Late [[Devonian]]{{efn|If [[paraphyletic]] in relation to the rest of [[Gnathostomata]], then modern jawed vertebrates represent extant forms.}}
| image = Bothriolepis panderi.jpg
| image_caption = Fossil of ''[[Bothriolepis|Bothriolepis panderi]]'', an [[antiarch]] placoderm showing its [[caliper|caliper-like]] [[pectoral fin]]s
| image2 = Coccosteus reconstruction.png
| image2_caption = Life restoration of ''[[Coccosteus]]'', an [[arthrodire]] placoderm
| display_parents = 2
| taxon = Placodermi
| authority = [[Frederick McCoy|McCoy]], 1848
| subdivision_ranks = Orders
| subdivision = * [[extinction|†]]'''[[Antiarchi]]'''
* †'''[[Arthrodira]]''' (includes [[Phyllolepida]])
* †[[Petalichthyida]]
* †[[Ptyctodontida]]
* †[[Rhenanida]]
* †[[Acanthothoraci]] ([[paraphyletic]])
* "Maxillate placoderms"
** †''[[Qilinyu]]''
** †''[[Entelognathus]]''
** †''[[Silurolepis]]''
** †''[[Bianchengichthys]]''
** †''[[Minjinia]]''
** ?[[Eugnathostomata]]
* †''[[Xiushanosteus]]''
* †''[[Brindabellaspis]]''
* ?†[[Pseudopetalichthyida]]
* ?†''[[Stensioella]]''
| synonyms = * Placodermata
}}

'''Placoderms''' (from [[Ancient Greek]] πλάξ [''plax'', ''plakos''] '[[Plate (animal anatomy)|plate]]' and δέρμα [''derma''] 'skin')<ref>{{cite book |last1=Colbert |first1=Edwin H. (Edwin Harris) |last2=Knight |first2=Charles Robert |title=The dinosaur book: the ruling reptiles and their relatives |date=1951 |publisher=McGraw-Hill |location=New York |page=153 |url=https://archive.org/details/bookruli00colb/page/152/mode/2up}}</ref> are [[vertebrate]] [[animal]]s of the [[class (biology)|class]] '''Placodermi''', an [[extinct]] group of [[prehistoric fish]] known from [[Paleozoic]] [[fossil]]s during the [[Silurian]] and the [[Devonian]] [[geological period|period]]s. While their [[endoskeleton]]s are mainly [[cartilaginous]], their [[head]] and [[thorax]] were covered by articulated [[armour (zoology)|armoured plate]]s (hence the name), and the rest of the body was [[scale (zoology)|scaled]] or naked depending on the [[species]].

Placoderms were among the first [[jawed fish]] (their [[fish jaw|jaws]] likely [[Evolution|evolve]]d from the first pair of [[gill arch]]es), as well as the first vertebrates to have true [[tooth|teeth]]. They were also the first fish [[clade]] to develop [[pelvic fin]]s, the second set of [[paired fins]] and the [[homology (biology)|homologous]] precursor to [[hindlimb]]s in [[tetrapod]]s.<ref name="Rucklin2012">{{Cite journal | last1 = Rücklin | first1 = M. | last2 = Donoghue | first2 = P. C. J. | last3 = Johanson | first3 = Z. | last4 = Trinajstic | first4 = K. | last5 = Marone | first5 = F. | last6 = Stampanoni | first6 = M. | title = Development of teeth and jaws in the earliest jawed vertebrates | doi = 10.1038/nature11555 | journal = Nature | volume = 491 | issue = 7426 | pages = 748–751 | year = 2012 | pmid = 23075852| bibcode = 2012Natur.491..748R | s2cid = 4302415 }}</ref> 380-million-year-old fossils of three other genera, ''[[Incisoscutum]]'', ''[[Materpiscis]]'' and ''[[Austroptyctodus]]'', represent the oldest known examples of [[viviparity|live birth]].<ref name="BBC">{{cite news | publisher = BBC | url = http://news.bbc.co.uk/2/hi/science/nature/7424281.stm | title = Fossil reveals oldest live birth | date = May 28, 2008 |access-date = May 30, 2008 }}</ref>

Placoderms are thought to be [[paraphyly|paraphyletic]], consisting of several distinct [[outgroup (cladistics)|outgroup]]s or [[sister taxon|sister taxa]] to all living jawed [[vertebrate]]s, which originated among their ranks.<ref>{{cite journal |last1=Li |first1=Qiang |last2=Zhu |first2=You-an |last3=Lu |first3=Jing |last4=Chen |first4=Yang |last5=Wang |first5=Jianhua |last6=Peng |first6=Lijian |last7=Wei |first7=Guangbiao |last8=Zhu |first8=Min |title=A new Silurian fish close to the common ancestor of modern gnathostomes |journal=Current Biology |date=August 2021 |volume=31 |issue=16 |pages=3613–3620.e2 |doi=10.1016/j.cub.2021.05.053 |pmid=34146483 |s2cid=235477130 |doi-access=free |bibcode=2021CBio...31E3613L }}</ref> In contrast, one 2016 analysis concluded that Placodermi is likely [[monophyletic]].<ref>{{cite journal |last1=King |first1=Benedict |last2=Qiao |first2=Tuo |last3=Lee |first3=Michael S. Y. |last4=Zhu |first4=Min |last5=Long |first5=John A. |title=Bayesian Morphological Clock Methods Resurrect Placoderm Monophyly and Reveal Rapid Early Evolution in Jawed Vertebrates |journal=Systematic Biology |date=5 December 2016 |volume=66 |issue=4 |pages=499–516 |doi=10.1093/sysbio/syw107 |pmid=27920231 |doi-access=free }}</ref>

The first identifiable placoderms appear in the [[fossil record]] during the late [[Llandovery epoch]] of the early Silurian.<ref name="BurrowTurner">{{cite journal | title=A review of placoderm scales, and their significance in placoderm phylogeny |author1=Burrow, Carol |author2=Turner, Susan |name-list-style=amp | journal=Journal of Vertebrate Paleontology |date=September 1998 | volume=19 | issue=2 | pages=204–219 | doi=10.1080/02724634.1999.10011135}}</ref> They eventually outcompeted the previously [[dominance (ecology)|dominant]] marine [[arthropod]]s (e.g. [[eurypterid]]s) and [[cephalopod]] [[mollusc]]s (e.g. [[orthocone]]s), producing some of the first and most infamous vertebrate [[apex predator]]s such as ''[[Eastmanosteus]]'', ''[[Dinichthys]]'' and the massive ''[[Dunkleosteus]]''. Various groups of placoderms were diverse and abundant during the Devonian, but all placoderms became extinct at the end-Devonian [[Hangenberg event]] 358.9 million years ago,<ref name="SallanCoates">{{cite journal | title=End-Devonian extinction and a bottleneck in the early evolution of modern jawed vertebrates |author1=Sallan, Lauren |author2=Coates, Michael |name-list-style=amp | journal=Proceedings of the National Academy of Sciences |date=June 2010 | volume=107 | pages=10131–10135| doi= 10.1073/pnas.0914000107 | pmid=20479258 | issue=22 | pmc=2890420| bibcode=2010PNAS..10710131S |doi-access=free }}</ref> leaving the [[ecological niche|niche]]s open for the [[osteichthyan]] and [[chondrichthyan]] survivors who subsequently [[evolutionary radiation|radiate]]d during the [[Carboniferous]].